RFR: Avoid running PullRequestBot WorkItems for the same PR concurrently

[Robin Westberg via github.com]

Hi all,

Please review this change that ensures that the WorkItems used by the PullRequestBot are not run concurrently for the same PR. This avoids a race condition where two tasks try to update the `sponsor` label at the same time.

Best regards,
Robin

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Commits:
 - b3319650:	Avoid running workitems for the same PR concurrently

Pull request:
https://git.openjdk.java.net/skara/pull/106

Webrev:
https://webrevs.openjdk.java.net/skara/106/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/106.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/106/head:pull/106